1. Field of the Invention
The present invention relates to power converters and voltage regulators, and more particularly, to a switching and linear voltage regulator for providing power to a noise sensitive circuit.
2. Description of the Prior Art
Voltage regulators are basic building blocks used in electronic circuit design, and are commonly found in many applications including laptop computers, personal digital assistants (PDAs), cell phones, cable modems and battery-operated electronic devices. Voltage regulators are used to provide and maintain a constant DC output source regardless of changes in load current and input voltage (assuming the values stay within the specification of the regulator). Because most electronic circuits are designed to operate at a constant supply voltage, the use of a voltage regulator is critical to ensure proper operation of the electronic device. A voltage regulator can also help protect a circuit from variations in input voltage, temperature, and load impedance.
Although there are many types of voltage regulators in existence, many of which cater to a specific application, most voltage regulators fall to within one of two categories, linear and switching.
FIG. 1 displays a typical block diagram of a linear regulator 100, generally the most common type of voltage regulators. The linear regulator 100 makes use of a voltage controlled current source 140 to force a fixed voltage across an output terminal 120. An input voltage 110 is applied to the regulator 100, while a control circuit is used to monitor the output voltage at output terminal 120, and to adjust the voltage controlled current source 140 to maintain the output voltage according to a reference voltage 130.
Popularity of the linear regulator can be attributed to many things, they are low cost, simple, have low output noise characteristics, and have fast response times to load variations. However, linear regulators are not electrically efficient and can have significant heat losses. This is especially important in devices where battery life is critical, such as cell-phones and portable electronics.
FIG. 2 illustrates a conventional switching regulator 200 of the related art. A basic configuration of this type makes use of a switch 210, a pulse width modulator 220 to control switch 210, and an LC circuit consisting of an inductor 230 and capacitor 240. When switch 210 is turned on, input voltage 250 is forced across inductor 230 resulting in an inductor current. This will, according to the law of inductance, cause the voltage across the inductor to increase with proportion to the rate of current across it. The inductor current acts to charge capacitor 240, and supply current into the output terminal 260. The output voltage is compared to a voltage reference 270, and this feedback is used to vary the pulse width application of power to output 260. In this manner, the circuit acts as a constant current source supplying current into the load. The desired voltage level and ripple can be selected by properly adjusting the frequency of the pulse width modulator 220.
Switching regulators typically possess high efficiencies, are capable of handling high energy densities, and have less heat losses when compared to linear regulators. However, they tend to have higher output voltage ripples and produce more noise and electromagnetic interference. In devices that are sensitive to output noise from power sources (RF communications, wireless devices), this may adversely interfere with device operations.
Therefore, for applications such as RF communications and portable wireless devices, it is desirable to have power supply regulators that have high efficiency, and low output noise to ensure proper device operation and long battery life.